The present invention relates to a leaf spring unit employed in a printing head of a dot matrix printer, and more particularly to a leaf spring having a print wire for a printer of the type in which the spring is prestressed by a permanent magnet.
FIG. 1 shows a conventional leaf spring unit disclosed, for example in U.S. Pat. No. 4,348,120. The leaf spring unit comprises a leaf spring 1 cantilevered on a supporting member 2, an armature 3 secured to a free end portion 1a of the leaf spring 1, and a print wire 4 mounted on the armature 3. A core 5 having an exciting coil (not shown) is disposed adjacent to the free end portion 1a of the leaf spring 1 at a rear side thereof.
At a waiting position, the armature 3 secured to the leaf spring 1 is attracted to the core 5 by magnetic force of a permanent magnet (not shown) against the elasticity of the leaf spring 1. When the coil provided on the core 5 is excited, the flux of the permanent magnet is canceled so that the print wire 4 is driven forward by the elastic force of the leaf spring 1 to print a dot. Thus, the leaf spring 1 is reciprocated between the printing position and the reset position.
Since the armature 3 has a large thickness, an end part of the leaf spring 1 where the armature is secured acts as a stiff arm and a residual portion 1b acts as a flexible arm. Thus, a border portion 1c adjacent the armature or an end portion of the flexible portion 1b is greatly bent during the operation. In other words, the cross section of the bent portion largely changes in area. This causes a concentration of stress to the border portion 1c. Further, when the print wire 4 impacts a platen (not shown), the border portion 1c overreaches the nuetral position as shown in dotted-dashed line in FIG. 1, because of a large mass in a portion around the border portion 1c. Accordingly, excessive tensile stress develops in the portion 1c. As a result, the leaf spring 1 is broken after being in use for a short period.
FIG. 2 shows another leaf spring unit in which a leaf spring 6 is engaged with a groove 7a provided in an armature 7. However, end surfaces 7b, 7c of the armature 7 are in line with each other. Therefore, large tensile stress also develops in a border portion 6c.